The present invention relates to the measurement of workpieces using a surface measuring probe.
Measurement probes are known which have a stylus which is deflectable when acted upon by a force as a result of contact between the stylus and the surface of an object. One or more transducers within the probe measure the deflection of the stylus (usually in three orthogonal coordinate directions) in order to obtain information about the position of the surface. In use, such a probe is mounted in a machine such as a coordinate measuring machine (CMM), machine tool, measuring robot or other coordinate positioning apparatus. The machine moves the probe around the object to be measured. Measuring devices in the machine give outputs relating to the position of the probe, which when combined with the outputs from the probe itself enable information to be obtained about the size, shape, position, surface contours, etc of the object.
Such a measuring probe may be referred to as an xe2x80x9canalogue probexe2x80x9d, distinguishing the measurement outputs of its transducers from probes which merely produce a trigger signal upon contact with an object. The term xe2x80x9cscanning probexe2x80x9d may also be used, since such probes are often used for scanning the surface contour of an object. Even though the term xe2x80x9canalogue probexe2x80x9d may be used, the outputs of the transducers may in fact be analogue or digital.
In known systems, measurement errors are caused by deflections of the probe and machine structure. For example, when the stylus is deflected, it is de-biased towards a zero position e.g. by springs, and these forces cause bending of the probe stylus and of structural components of the machine. Whilst small, such deflections can nevertheless affect the accuracy of measurement, in view of the extremely high accuracies nowadays demanded.
Our earlier International Patent Application WO92/20996 describes a method of measurement in which such a probe is moved into contact with the surface of an object to be measured, and the movement is continued for a further limited distance after initial contact has been made. During this movement, the outputs of the measuring devices of the machine and the transducers of the probe are simultaneously recorded at a plurality of instants. These recorded outputs are then used to compute, by extrapolation, the values of the outputs of the measuring devices of the machine which were existing at the instant that the probe stylus was in a state of zero deflection and still in contact with the surface.
This method allows an analogue probe to be used as if it were a very accurate touch trigger probe since the value of the machine""s outputs is determined at the point where the stylus contacts the surface, in the same way as a trigger probe. The higher accuracy of this method derives partly from the fact that many data points are taken in order to determine the point of contact, so that errors tend to be averaged out. Furthermore, a particular advantage is that the contact point determined corresponds to zero deflection of the stylus, and consequently zero contact force between the stylus and the object, so that errors due to bending of the stylus and/or of the machine structure do not arise.
A first aspect of the present invention provides a method of measuring an artefact using a machine on which a measuring probe is mounted for relative movement with respect to the artefact, said machine having at least one measuring device for providing an output indicative of the relative position of the probe, the probe having a deflectable stylus, and at least one measuring device for measuring deflections of the stylus to provide one or more probe outputs which are indicative of the amount of deflection of the stylus from a rest position, the method being characterized by the steps of:
causing relative movement between the probe and artefact to bring the stylus into contact with the surface of an artefact and continuing said movement for a limited distance after initial contact has been made between the stylus and the artefact;
recording the outputs of the machine and of the probe at a plurality of instants, during both the periods of contact and non contact between the stylus and the artefact;
providing a model of the probe and CMM outputs which models the outputs both during contact and non contact between the stylus and the artefact;
fitting the model to the data and thereby determining the values of the outputs of the measuring device or devices of the machine relating to the contact position at which the stylus contacts the artefact with zero contact force; and
combining said outputs of the measuring device or devices of the machine at said contact position with the probe deflection at the contact instant to establish the contact position at zero force.
The outputs of the measuring devices of the machine and the probe may be recorded during one or both of the probes relative movement towards the artefact and away from the artefact.
Preferably the model is a break line. The break line may comprise a line of zero gradient and at least one sloped line.
Preferably the values of the outputs of the machine when the stylus contacts the artefact with zero contact force are determined by an optimisation procedure.
A second aspect of the present invention provides a method of measuring an artefact using a machine on which a measuring probe is mounted for relative movement with respect to the artefact, said machine having at least one measuring device for providing an output indicative of the relative position of the probe, the probe having a deflectable stylus and at least one measuring device for measuring deflections of the stylus to provide two or more probe outputs which are indicative of the amount of deflection of the stylus from a rest position along at least two probe axes, the method including the steps of:
causing relative movement between the probe and artefact to bring the stylus into contact with the artefact and continuing said movement for a limited distance after initial contact has been made between the stylus and the artefact;
recording the outputs of the machine and of the probe at a plurality of instants, in at least part of the period of contact between the stylus and the artefact;
from said outputs of the machine and of the probe, determining the values of the outputs of the machine relating to the contact position when the stylus contacts the artefact with zero contact force;
wherein the data from each probe output is used individually to determine a single contact position.
A third aspect of the present invention provides a method of measuring an artefact using a machine on which a measuring probe is mounted for relative movement with respect to the artefact, said machine having at least one measuring device for providing an output indicative of the relative position of the probe, the probe having a deflectable stylus and at least one measuring device for measuring deflections of the stylus to provide one or more probe outputs which are indicative of the amount of deflection of the stylus from a rest position, the method including the steps of:
causing relative movement between the probe and the artefact to bring the stylus into contact with the artefact and continuing said movement for a limited distance after initial contact has been made between the stylus and the artefact;
recording the outputs of the machine and of the probe at a plurality of instants, in at least part of the period of contact between the stylus and the artefact, both for relative movement between the probe and artefact towards and away from one another;
from said outputs of the machine and of the probe, determining the values of the outputs of the machine relating to the contact position when the stylus contacts the artefact with zero contact force, both for relative movement between the probe and artefact towards and away from one another, thereby determining two apparent contact positions;
and wherein the true contact position is determined by combining the two apparent contact positions.
The true contact position is thereby corrected for time delays.
The combination of the two apparent contact positions may comprise taking an average of the two apparent contact positions.
The average may be weighted by the relative speeds of the machine during the relative movement between the probe and the artefact in each direction.
A fourth aspect of the invention comprises a method of measuring an artefact using a machine on which a measuring probe is mounted for relative movement with respect to the artefact, said machine having at least one measuring device for providing an output indicative of the relative position of the probe, the probe having at least one measuring device for measuring the distance from a surface of the artefact to provide one or more probe outputs, the method including the steps of:
causing relative movement between the probe and the artefact;
recording the outputs of the machine and of the probe at a plurality of instants, both for relative movement between the probe and artefact towards and away from one another;
from said outputs of the machine and of the:probe, determining two apparent surface positions of a surface of the artefact, the two apparent surface positions relating to relative movement between the probe and artefact towards and away from one another respectively;
and wherein the true surface position is determined by combining the two apparent surface positions.
The true contact position is thereby corrected for time delays.
The combination of the two apparent surface positions may comprise taking an average of the two apparent surface positions.
The average may be weighted by the relative speeds of the machine during the relative movement in between the probe and artefact in each direction.